In recent years, from the viewpoint of prevention of global warming, energy saving is strongly demanded, and the energy saving is an urgent problem in electric household appliances. Especially, in a refrigerator, a freezer, or a warm and cold storage such as a vending machine, a heat insulating material having an excellent heat insulating property is desired from the viewpoint of efficient use of heat.
As a general heat insulating material, fiber material such as glass wool or foamed body such as urethane foam is used. However, to enhance the heat insulating property of these heat insulating materials, it is necessary to increase a thickness of the heat insulator, and in a case where the size of the space to which the heat insulating material is filled is limited and therefore the space saving or effective use of the space is needed, the heat insulating material cannot be adopted.
Accordingly, a vacuum heat insulating material is proposed as a heat insulating material with high performance. The vacuum heat insulating material is a heat insulating body in which a core material having a function as a spacer is inserted into an enveloping member having a gas barrier property and sealed after evacuating the inside.
The vacuum heat insulating material has a heat insulating property that is about 20 times as efficient as the heat insulating property of urethane foam. Thus, the vacuum heat insulating material has such excellent characteristics that even when the thickness of the vacuum heat insulating material is reduced, sufficient heat insulating property can be obtained.
Accordingly, the vacuum heat insulating material satisfies customer needs to increase an inner volume of a heat insulating box. Moreover, the vacuum heat insulating material attracts attention as an effective member for saving energy by improving the heat insulating property.
Further, in the refrigerator or the like, by additionally disposing the vacuum heat insulating material between an inner box and an outer box of the heat insulating box which forms a refrigerator body and further filling with foamed urethane foam, the heat insulating property is enhanced and an inner volume of the heat insulating box is enlarged (for example, see PTL 1).
Further recently, the vacuum heat insulating material is considered for use in a freezing container which needs to store objects at a temperature lower than that of the refrigerator, or a tank for liquefied natural gas (LNG) or the like which needs to store objects at an ultra-low temperature further lower than that of the freezing container (for example, see PTL 2).
When the vacuum heat insulating body is adopted in the freezing container, the energy saving can be enhanced due to its high heat insulating property. Further, when the vacuum heat insulating body is adopted in a heat insulating container such as an LNG tank, heat invasion into the heat insulating container can be suppressed efficiently, and therefore generation of boil off gas (BOG) can be reduced.
However, since a cold storage temperature of the freezing container is approximately in a range from −60° C. to −25° C. and the temperature of LNG is the ultra-low temperature of −162° C., the vacuum heat insulating body having a heat insulating property as high as possible is required, and therefore it is desired to further improve the heat insulating property.
The present invention has been made in view of such circumstances, and an object thereof is to improve a heat insulating property of a vacuum heat insulating body which forms a heat insulating container or the like.